1. Field of the Invention
The present invention relates to a wireless communication system, and more particularly, to a method and apparatus for measuring a cell in a wireless communication system.
2. Discussion of the Related Art
With reference to FIG. 1, a Universal Mobile Telecommunications System (UMTS) network configuration will be described below.
FIG. 1 illustrates a UMTS network configuration. Referring to FIG. 1, a UMTS system includes a User Equipment (UE), a UMTS Terrestrial Radio Access Network (UTRAN), and a Core Network (CN). The UTRAN includes one or more Radio Network Sub-systems (RNSs) each having a Radio Network Controller (RNC) and one or more Node Bs managed by the RNC. A Node B manages one or more cells.
A radio protocol architecture for the UMTS system will be described with reference to FIG. 2. FIG. 2 illustrates a radio protocol architecture for UMTS. Radio protocol layers are defined in pairs for a UE and a UTRAN, for wireless data transmission. Layer 1 (or L1), the PHYsical (PHY) layer transmits data on a radio link in various wireless transmission techniques. The PHY layer is connected to its higher layer, the MAC layer via transport channels. The transport channels are divided into dedicated transport channels and common transport channels depending on whether they are shared.
The MAC layer, the Radio Link Control (RLC) layer, the Packet Data Convergence Protocol (PDCP) layer, and the Broadcast and Multicast Control (BMC) layer are defined at Layer 2 (or L2). The MAC layer maps logical channels to transport channels and multiplexes a plurality of logical channels onto one transport channel.
The MAC layer is connected to a higher layer, the RLC layer via logical channels. The logical channels are divided into control channels and traffic channels according to the types of information that they carry. The control channels carry control-plane information and the traffic channels carry user-plane information. The control channels include a Common Control Channel (CCCH) carrying common control information, a Dedicated Control Channel (DCCH) carrying control information to a specific UE, a Broadcast Control Channel (BCCH) carrying system information common to a cell, and a Paging Control Channel (PCCH) carrying a paging message. The traffic channels include a Dedicated Traffic Channel (DTCH) carrying user-plane data to a specific UE.
The MAC layer is branched into a MAC-b sublayer, a MAC-d sublayer, a MAC-c/sh sublayer, a MAC-hs/ehs sublayer, and a MAC-e/es or MAC-i/is sublayer depending on the types of specific transport channels that they manage. The MAC-b sublayer manages a Broadcast Channel (BCH) that broadcasts system information, the MAC-c/sh sublayer manages a Forward Access Channel (FACH) that is a common transport channel shared among different UEs, and the MAC-d sublayer manages a Dedicated Channel (DCH) that is a dedicated transport channel for a specific UE. The MAC-hs/ehs sublayer manages a High Speed Downlink Shared Channel (HS-DSCH) that is a transport channel used to transmit high-speed downlink data, and the MAC-e/es or MAC-i/is sublayer manages an Enhanced Dedicated Channel (E-DCH) that is a transport channel used to transmit high-speed uplink data.
The RLC layer ensures the Quality of Service (QoS) of Radio Bearers (RBs) and is responsible for data transmission. The RLC layer has one or two independent RLC entities for each RB in order to ensure QoS. To support various QoS levels, the RLC layer provides three RLC modes, Transparent Mode (TM), Unacknowledged Mode (UM), and Acknowledged Mode (AM). In addition, the RLC layer controls a data size to suit radio data transmission at a lower layer. For controlling a data size, the RLC layer segments or concatenates data received from a higher layer.
The PDCP layer is located above the RLC layer. The PDCP layer enables efficient data transmission in IP packets such as IP version 4 (IPv4) or IP version 6 (IPv6) packets on a radio link having a relatively narrow bandwidth. For this purpose, the PDCP layer performs header compression. Since only necessary information is transmitted in the header of data header through header compression, the transmission efficiency of the radio link is increased. The PDCP layer exists mainly in a Packet Switched (PS) domain because header compression is it basic function. To provide an efficient header compression function for each PS service, one PDCP entity is defined for each RB. However, if the PDCP layer exists in a Circuit Switched (CS) domain, the PDCP layer does not provide the header compression function.
The BMC layer is also above the RLC layer, for scheduling a cell broadcast message and broadcasting the cell broadcast message to UEs within a specific cell.
The Radio Resource Control (RRC) layer, which is located at the lowest part of Layer 3 (or L3), is defined only on the control plane. The RRC layer is involved in establishing, reestablishing, and releasing RBs, controls L1 or L2 parameters, and controls logical channels, transport channels and physical channels. An RB refers to a logical path formed at L1 and L2 in the protocol stack, for data transmission between a UE and a UTRAN. In general, setup of an RB is the process of specifying radio protocol layers and channels necessary to provide a specific service and setting specific parameters and an operation scheme.
Now a description is given of dual cell High Speed Packet Access (HSPA) of UMTS and Carrier Aggregation (CA) of Long Term Evolution-Advanced (LTE-A).
Compared to the existing E-DCH on which a UE transmits data at a single frequency, dual cell HSPA doubles the amount of transmitted data by allowing a UE to transmit data simultaneously at two frequencies. An operation of transmitting data at two frequencies from the UE is referred to as a dual cell E-DCH operation. Conventionally, a UE receives a High Speed Downlink Shared Channel (HS-DSCH) at a single frequency. In contrast, an operation of simultaneously receiving an HS-DSCH at two frequencies at a UE to thereby double the amount of received data is referred to as a dual cell HSDPA operation.
In the LTE-A system, studies are being conducted on defining a carrier used in a legacy LTE system as a Component Carrier (CC) and grouping up to five CCs in order to extend a bandwidth. This technology is called CA.
Conventionally, when a UE transmits or receives data to or from a Base Station (BS) at a plurality of frequencies as is done in dual cell HSPA and CA, the UE accesses a cell in idle mode and then a network establishes an RRC connection with the UE, for one frequency. If the UE is in RRC connected state, the network transmits measurement configuration information for a plurality of frequencies to the UE. Then the UE performs cell measurement on the plurality of frequencies based on the received measurement configuration information and reports the cell measurements to the network. The BS configures a plurality of frequencies for the UE using the received cell measurements. Because the network does not configure a plurality of frequencies immediately after the idle-mode UE accesses the cell, the UE transmits or receives data at a high data rate on the plurality of frequencies after a certain time delay. Therefore, the conventional technology experiences a time delay in configuring a plurality of frequencies between a UE and a network.